This invention relates to melt-processible fluoropolymer rendered conductive by the incorporation of carbon black.
Fluoropolymers are used in containers and hoses for flammable fluids because they are little affected by such fluids and also are highly impermeable to them. Because the containers and hoses should have some electrical conductivity to prevent the accumulation of static charge, which can cause safety problems, fillers are added to the fluoropolymers to confer electrical conductivity. To obtain articles having uniform electrical conductivity, it is desirable that fillers that impart conductivity be uniformly distributed.
Japanese Kokai Patent Hei 11 (1999)-35694 discloses a method for granulation of melt-processible fluoropolymer by the addition of polyfluoroalkyl alkyl ether to aqueous fluorocarbon dispersion. The resultant fluoropolymer powder granules have a large average particle size (1.0 to 1.5 mm according to the Examples). Addition of conductive filler to these granules leads to uneven distribution of the filler and this adversely affects conductivity of articles melt-fabricated from the granule/filler mixture.
Japanese Kokai Patent 2000-103865 discloses a method to obtain an improved fluoropolymer powder. After melt-processible fluoropolymer obtained from the solution polymerization or suspension polymerization is coagulated and granulated, the granules are ground and heat-treated above the melting point of the polymer to melt at least some of particles thereby obtaining a fluoropolymer powder. The average particle size of the resultant fluoropolymer powder is as large as 0.5 to 5.0 mm, i.e. the grinding of these granules generally does not reduce the average particle size of the granules to less than 0.5 mm (500 xcexcm). The grinding of fluoropolymer granules has the further disadvantage of being an expensive operation. Mixing of conductive filler with these ground granules yields compositions, which when melt-fabricated, give articles in which the conductive filler is not uniformly distributed.
Another method for obtaining a fluoropolymer/carbon black composition involves mixing tetrafluoroethylene/perfluoro(alkyl vinyl ether) (PFA) with acetylene black as a conducting carbon black in a Henshel mixer (U.S. Pat. No. 5,106,539). Still another method involves melt-mixing a terminal-group-fluorinated PFA with a conducting carbon black for reduced electrical resistance and a minimized increase in relative viscosity (U.S. Pat. No. 5,005,875).
However, these fluoropolymer/carbon black compositions have not shown satisfactory properties for use in electronic and electrical devices and parts applications that require smooth and high-precision surfaces, uniform electrical conductivity and little or no contamination, i.e. little or no release of the carbon black into the use environment, e.g. process liquids used in the manufacture of semi-conductors.
The electrical resistance of conducting fluoropolymer compositions is known to vary not only with the type and amount of the conducting carbon black mixed therewith, but also greatly with the way the conducting carbon black is dispersed (Journal of Applied Polymer Science Vol. 69, p.193 (1998)). However, with the mixing methods described in the patent specifications cited above, which call for mixing PFA particles with conducting carbon black in a Henshel mixer and then mixing in a melt extruder, or for forcing the PFA pellets to be melt mixed with conducting carbon black by the shearing force of the extruder, it has been difficult to disperse the mixed fine conducting carbon black powder uniformly in the PFA resin. In particular, melt-mixing the conducting carbon black with the melt processible fluoropolymer in an extruder causes simultaneous dispersion of the conducting carbon black and destruction of the structure of the carbon black during the melt mixing state, making it extremely difficult for the composition to provide controlled stable conductivity. Because the conducting carbon black is dispersed non-uniformly, there is considerable variation in the conductivities and the physical properties within the same manufactured batch, or between batches, with the formation of agglomerates of non-uniformly dispersed carbon black and/or undispersed fluoropolymer. This has made it difficult to obtain a molded article with a smooth surface and also has made it impossible to obtain a shaped article product, which when used in a wafer holding jig or solvent line employed in semi-conductor manufacturing devices, would not release the conducting carbon black particles into the liquid of the device.
There is a need for a process for improved mixing of fluoropolymer and carbon black to give a composition that can be melt-fabricated into articles having uniform conductivity.
The present invention satisfies this need by providing a method for producing a uniform mixture of carbon black in melt-processible fluoropolymer, comprising pulverizing a composition comprising (a) said carbon black and (b) said melt-processible fluoropolymer, said carbon black in said composition being in the form of aggregates of primary particles and said melt-processible fluoropolymer in said composition being in the form of coagulum particles of aqueous dispersion polymerized melt-processible fluoropolymer, the pulverizing of said composition causing the disintegration of said aggregates and the coating of said coagulum particles with the disintegrated particles, thereby obtaining said uniform mixture.
In another embodiment, the present invention relates to a method for producing a melt-processible fluoropolymer powder having an average particle size of no more than 10 xcexcm, comprising stirring an aqueous dispersion polymerization medium containing said fluoropolymer polymerized in said medium, said medium also containing electrolyte, to form a coagulum of said fluoropolymer, separating said coagulum from the resulting medium, drying said coagulum, and then pulverizing said dried coagulum together with carbon black having a primary particle size of no more than about 0.1 xcexcm, and obtaining said powder as a uniform mixture of said carbon black in said fluoropolymer.
The resultant powder can be melt-fabricated into articles that have uniform electrical conductivity.
Use of a particular type of electrically conducting carbon black, i.e. one which has a developed structure (fine powder aggregates of very small primary particles as described hereinafter) provides a melt fabricable electrically conducting fluoropolymer composition which exhibits a unique DSC fingerprint. Thus such composition comprises conducting carbon black dispersion in a melt processible fluoropolymer wherein said melt processible fluoropolymer has two crystallization peaks as determined by differential scanning calorimeter (DSC) when said composition is allowed to crystallize from a temperature of not less than its melting point at a cooling rate of 12xc2x0 C./min; said two crystallization peaks consisting of a high temperature side peak and a low temperature side peak, and wherein the ratio of the crystallization peak heights (high temperature side peak/low temperature side peak) is 0.65 or greater; and/or the ratio of the high temperature side crystallization peak area to the total area of the crystallization peaks [high temperature side peak area/(high temperature side peak area+low temperature side peak area)] is 0.18 or greater.
Whereas the process of the present invention in general provides electrically conductive fluoropolymer compositions, which when melt fabricated, provide articles of high surface smoothness and low surface resistivity, characteristic of dispersion uniformity, the compositions of the present invention provide further improvement in dispersion uniformity, characterized by the additional improvements of both high surface smoothness and low surface resistivity, together with more intimate association of the carbon black with the fluoropolymer as indicated by articles melt fabricated from the composition not releasing carbon black particles into process liquids which come into contact with the molded article in use. Most electrically conducting carbon blacks do not provide the DSC fingerprint described above and the improvements just noted. Acetylene black has been found to provide both. It is contemplated that other electrically conducting carbon blacks exist that will act like acetylene black, particularly those that have structure like acetylene black, i.e. fine powder of very small primary carbon black particles. These aggregates disintegrate during the pulverizing process to much smaller aggregates, enabled by the fineness of the primary particles, to further improve the uniformity of the dispersion of the carbon black into and onto the fluoropolymer fine powder present in the pulverizing process.